Method for stamping thermally activatable adhesive materials which are not tacky at room temperature

ABSTRACT

The invention relates to a method for producing stampings from thermally activatable adhesive films, said method being characterised in that a) a thermally activatable adhesive material is first applied to a first carrier material, and b) the thermally activatable layer of adhesive material is laminated onto a temporary carrier material (“release liner”) under the action of heat, during a subsequent step. The release liner comprises at least one surface based on polyolefins, said surface being non-tacky at room temperature and activatable under the action of heat in such a way that it has adhesive properties. The method is also characterised in that c) the thermally activatable layer of adhesive material on the release liner is subjected to a stamping process.

The invention relates to a method of producing stamped products(diecuts) from heat-activatable adhesives, and also to the use ofinnovative release liners.

Heat-activatable adhesives very often replace pressure-sensitiveadhesive tapes in operations where the tapes are required to withstandhigh temperatures or where very high bond strengths are needed.

Heat-activatable adhesives possess such properties because they have notack at room temperature and become soft and tacky only with anaccompanying increase in temperature. For the mediation of adhesion (forthe formation of an adhesive layer), heat-activatable adhesive sheetsrequire application of heat and in general—as for all pressure-sensitiveor self-adhesive materials—the application of a certain pressure. Ingeneral a distinction is made between thermoplastic and thermallyreactive heat-activatable adhesives.

Thermoplastic heat-activatable films are based on the concept that thesefilms are hard at room temperature and then, under heat and pressure,soften, flow on the substrate and then become solid again withaccompanying cooling. Thereafter the adhesion to the substrate to bebonded is high, as a result of the wetting process of the adhesive.

In the case of the thermally reactive heat-activatable adhesives, theprocess that operates is the same in analogy to the thermoplasticheat-activatable adhesives. Additionally here, though, in the course oftemperature activation, a crosslinking reaction is initiated whichresults in the thermally reactive heat-activatable adhesives curing, andhence leads to an increased bond strength.

As a result of the properties described above, heat-activatableadhesives are often used for bonds of metals, or of metals withplastics. This also concerns the electronics industry, with theconsequence that heat-activatable adhesives are being used increasinglyin the form of diecuts (products from stamping operations).

The production of such diecuts in diecutting processes, however, is notentirely trivial, for reasons including the fact that heat-activatablefilms cannot, like pressure-sensitive adhesives, simply betransfer-laminated to a desired liner, since at room temperature thepressure-sensitive adhesion is absent. This process generally does notfunction even with highly graduated release liners in conjunction with ahot-laminating operation.

Nevertheless, within the electronics industry, there is a need toproduce such diecuts; in particular to the exclusion of disruptive,unwanted paper fibers, or, for sensitive applications, without silicone.Since a very large number of heat-activatable adhesives are typicallyfurnished with glassine release liners, the aforementioned problem ismanifested to a particular degree. There is therefore a need for ageneral diecutting process for heat-activatable adhesives that operatesto the exclusion of paper release liners.

It is an object of the invention, therefore, to offer a method ofproducing diecuts from heat-activatable adhesives that avoids, or atleast minimizes, the disadvantages of the prior art. A particular aim isto avoid the presence of paper as a carrier material in the diecuttingoperation itself. Advantageously it is to be possible to optimize thepartability of the diecuts from the carrier material for the particularintended application.

The object is achieved by means of a specific production method for thediecuts, using specific release liners.

Accordingly the invention provides a method of processing aheat-activatable adhesive which is being coated onto a release liner,comprising an operation of hot transfer lamination onto one of theserelease liners, with a subsequent diecutting operation.

The main claim accordingly relates to a method of producing diecuts fromheat-activatable adhesive films, characterized in that

a) a heat-activatable adhesive is coated initially onto a first carriermaterial,

b) the heat-activatable adhesive layer, in a subsequent step of themethod, is laminated onto a temporary carrier material (release liner)with accompanying exposure to heat,

-   -   the release liner having at least one polyolefin-based surface,        and the polyolefinic surface being nontacky at room temperature        and being itself activatable, with accompanying exposure to        heat, in such a way that it has pressure-sensitive adhesive        properties,

c) and the heat-activatable adhesive layer on the release liner issubjected to a diecutting operation.

The dependent claims relate to developments of this method.

A further claim relates, moreover, to the use of a layer of aheat-activatable film having at least one polyolefin-based surface whichis nontacky at room temperature and is activatable, with accompanyingexposure to heat, in such a way that it has pressure-sensitive adhesiveproperties as a temporary carrier material (release liner) forheat-activatable adhesives.

Diecuts are converted (especially two-dimensional) adhesive film shapeswhich have been produced in a diecutting operation. The actual shape,size, and thickness of the diecuts is in principle not restricted, andmay be adapted for the particular intended application.

Release liners in the sense of this specification are temporary carriermaterials, i.e., layers of materials on which the heat-activatableadhesive is deposited temporarily and from which the adhesivethereafter—with greater or lesser ease—can be detached again withoutresidue. Release liners of this kind are known in particular as linermaterials of adhesive strips which are wound into a roll; in thatapplication, the release liners prevent the individual winds of theadhesive strip from sticking to one another permanently.

For the diecutting process it may additionally be of advantage if theheat-activatable film is additionally lined with a liner, such as a PETliner, for example. The hot-laminating operation softens thepolyolefinic side of the release liner, and a slight pressure-sensitiveadhesion is developed, which then results in the heat-activatable filmadhering to this layer. Surprisingly, on hot application, theheat-activatable adhesive can be laminated again outstandingly from therelease liner onto the desired substrate for bonding.

A further aspect of the invention are developments of the method of theinvention, in respect especially of techniques which can be usedoutstandingly for the laminating operation. Thus, for example, pressingrolls or pressure rolls with temperature-resistant rubberizing areparticularly suitable for the laminating operation.

Heat-Activatable Adhesives:

The heat-activatable adhesive used for producing the adhesive diecuts inthe method of the invention is to be characterized in that it has nointrinsic tack at room temperature and is therefore different fromconventional pressure-sensitive adhesives (PSAs). Another characteristicof the heat-activatable adhesive is that it develops tack only when heatis introduced, and thus then acts as an adhesive. Adhesives of this kinddiffer substantially from heat-activatable adhesives which arepressure-sensitively adhesive at room temperature, of the kind knownfrom the prior art.

A feature of the heat-activatable adhesive is that it becomes tacky withaccompanying exposure to temperature, and optionally with exposure topressure, and develops a high bond strength by solidification afterbonding and cooling. Depending on the chemical nature of theheat-activatable adhesives, they have different static glass transitiontemperatures T_(g,A) or melting points T_(m,A), which significantlydetermine the application temperature for bonding and for hot transferlamination.

The heat-activatable adhesives can have different polymer structures. Ina first preferred embodiment, poly(meth)acrylate-based heat-activatableadhesives are employed. Great preference is given to usingpoly(meth)acrylate adhesives which are composed of polymers made from atleast the following monomers a1) 70% to 100% by weight of acrylic estersand/or methacrylic esters and/or free acids thereof, with the followingformula

CH₂═CH(R₁) (COOR₂),

where R₁═H and/or CH₃ and R₂═H and/or aliphatic hydrocarbon chainshaving 1 to 30 C atoms.

Additionally it is possible, for preparing the polymers, to add,optionally, as monomers

a2) up to 30% by weight of olefinically unsaturated monomers havingfunctional groups,

in order to be able to vary the properties—especially the adhesive andviscosity properties—of the adhesives.

One very preferred procedure uses, in the sense of the monomers a1),acrylic monomers comprising acrylic and methacrylic esters with alkylgroups composed of 1 to 14 C atoms. Specific examples, without wishingto be restricted by this enumeration, are methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate,propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, n-pentylacrylate, n-hexyl acrylate, n-hexyl methacrylate, n-heptyl acrylate,n-octyl acrylate, n-nonyl acrylate, lauryl acrylate, stearyl acrylate,stearyl methacrylate, behenyl acrylate, and their branched isomers, suchas 2-ethylhexyl acrylate, for example. Further classes of compound to beused, which may likewise be added in small amounts under a1), arecyclohexyl methacrylates, isobornyl acrylate, and isobornylmethacrylates.

One advantageous variant uses for a2) acrylic monomers corresponding tothe following general formula

where R₁ is H and/or CH₃ and the radical —OR₂ represents or constitutesa functional group which supports subsequent UV crosslinking of the PSA,which, for example, in one particularly preferred version, possesses anH-donor effect.

Particularly preferred examples for the component a2) are hydroxyethylacrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate,hydroxypropyl methacrylate, allyl alcohol, maleic anhydride, itaconicanhydride, itaconic acid, acrylamide and glyceridyl methacrylate, benzylacrylate, benzyl methacrylate, phenyl acrylate, phenyl methacrylate,t-butylphenyl acrylate, t-butyl-phenyl methacrylate, phenoxyethylacrylate, phenoxy-ethyl methacrylate, 2-butoxyethyl methacrylate,2-butoxyethyl acrylate, dimethylaminoethyl methacrylate,dimethylaminoethyl acrylate, diethyl-aminoethyl methacrylate,diethylaminoethyl acrylate, cyanoethyl methacrylate, cyanoethylacrylate, glyceryl methacrylate, 6-hydroxyhexyl methacrylate,N-tert-butylacrylamide, N-methylolmethacrylamide,N-(butoxy-methyl)methacrylamide, N-methylolacrylamide,N-(ethoxy-methyl)acrylamide, N-isopropylacrylamide, vinylacetic acid,tetrahydrofurfuryl acrylate, β-acryloyloxy-propionic acid,trichloroacrylic acid, fumaric acid, crotonic acid, aconitic acid,dimethylacrylic acid; this enumeration should not be taken as beingconclusive.

In a further preferred embodiment use is made, for the component a2), ofaromatic vinyl compounds, it being possible for the aromatic nuclei tobe composed preferably of C₄ to C₁₈ building blocks and also to containheteroatoms. Particularly preferred examples are styrene,4-vinylpyridine, N-vinylphthalimide, methylstyrene,3,4-dimethoxystyrene, 4-vinylbenzoic acid; this enumeration is notconclusive.

For the polymerization the monomers are chosen such that the resultingpolymers can be used as heat-activatable adhesives.

For these applications the glass transition temperature of the resultingpolymer is above room temperature, advantageously above 30° C.

For purely crystalline systems there is a thermal equilibrium betweencrystal and liquid at the melting point T_(m,A). Amorphous or partlycrystalline systems, in contrast, are characterized by thetransformation of the more or less hard, amorphous or partiallycrystalline phase into a softer (rubberlike to viscous) phase. At theglass point, particularly in the case of polymeric systems, there is a“thawing” (or “freezing” in the case of cooling) of the Brownianmolecular motion of relatively long chain segments.

The transition from the melting point T_(m,A) (which is actually definedonly for purely crystalline systems; “polymer crystals”) to the glasstransition point T_(g,A) can therefore be regarded as being fluid,depending on the fraction of the partial crystallinity of the sampleunder investigation.

The context of this specification, and in the sense of the observationsabove, the designation “glass transition temperature” and the specifyingof the corresponding numerical values encompasses not only the staticalglass transition temperature but also the melting point, depending onthe nature of the system under investigation.

The values given for the glass transition temperature (static glasstransition temperature, melting point) in this specification refer toits determination by means of calorimetric measurement (DSC; dynamicscanning calorimetry (dynamic differential calorimetry); cf. inparticular DIN 53765: 1994-03).

In accordance with the remarks above, for the purpose of obtaining apolymer glass transition temperature T_(g,A) ≧30° C., the monomers arevery preferably selected, and the quantitative composition of themonomer mixture advantageously chosen, such that the desired T_(g,A) forthe polymer is produced in accordance with equation (E1), in analogy tothe Fox equation (cf. T. G. Fox, Bull. Am. Phys. Soc. 1 (1956) 123).

$\begin{matrix}{\frac{1}{T_{g}} = {\sum\limits_{n}\frac{w_{n}}{T_{g,n}}}} & ({E1})\end{matrix}$

In this equation, n represents the serial number of the monomers used,w_(n) the mass fraction of the respective monomer n (% by weight), andT_(g,n) the respective glass transition temperature of the homopolymerformed from the respective monomers n, in K.

The various preparation processes (polymerization processes) forpolyacrylate adhesives of this kind are described later on below.

In a further preferred procedure, adhesives based on polyolefins,especially poly-α-olefins, are used for the heat-activatable adhesive.

The polyolefin-based heat-activatable adhesives preferably have glasstransition temperatures of +75° C. to 180° C. The bond strength of thesepolymers can be increased by deliberate additization. Thus, for example,polyimine copolymers or polyvinyl acetate copolymers can be used as bondstrength promoter additives.

To achieve the desired glass transition temperature, the monomersemployed, and also their quantities, are again preferably chosen, hereas well, such that the desired temperature results when equation (E1) isapplied.

For a practical application it is advantageous to further restrict theglass transition temperature of the heat-activatable adhesive. If thetemperature were to be too low, the risk would be that, in the course ofdispatch or during transport, the adhesive tape would begin to melt atelevated temperatures, and the adhesive tape would no longer be able tobe unwound. In order to find an optimum range it is possible to vary themolecular weight and also the comonomer composition of the monomers whenpreparing the adhesives. In order to set a low glass transitiontemperature, polymers having a medium or low molecular weight are used.In addition, low molecular weight and high molecular weight polymers canbe mixed with one another. Particularly preferred procedures usepolyethenes, polypropenes, polybutenes, polyhexenes or copolymers ofpolyethene, polypropene, polybutene or polyhexene.

Polethylene and polyethylene copolymers can be coated in the form, forexample, of aqueous dispersions. The mixture used is dependent in turnon the desired glass transition temperature of the heat-activatableadhesive.

Available commercially from the company Degussa under the trade nameVestoplast™ are various heat-activatable poly-α-olefins. Propene-richgrades are offered under the names Vestoplast™ 703, 704, 708, 750, 751,792, 828, 888 or 891. These grades possess melting points T_(m,A) of 99to 162° C. In addition there are also butene-rich grades, availablecommercially under the names Vestoplast™ 308, 408, 508, 520, and 608.They possess melting points T_(m,A) of 84 to 157° C.

Further examples of heat-activatable PSAs are described in thespecifications U.S Pat. No. 3,326,741, U.S. Pat. No. 3,639,500, U.S.Pat. No. 4,404,246, U.S. Pat. No. 4,452,955, U.S. Ser. No. 44/043,345,U.S. Pat. No. 4,545,843, U.S. Pat. No. 4,880,683, and U.S. Pat. No.5,593,759. In these specifications there are likewise references tofurther temperature-activatable PSAs.

In a further variant embodiment of the invention the heat-activatableadhesive is composed of at least one elastomer component and at leastone reactive resin component. Elastomers used are, in particular,synthetic rubbers, such as polyvinyl butyral, polyvinyl formal, nitrilerubbers, nitrile-butadiene rubbers, hydrogenated nitrile-butadienerubbers, polyacrylate rubbers, chloroprene rubbers,ethylene-propylene-diene rubbers, methyl-vinyl-silicone rubbers,fluorosilicone rubbers, tetrafluoroethylene-propylene copolymer rubbers,butyl rubbers, and styrene-butadiene rubbers, for example.

Nitrile-butadiene rubbers are available for example as Europrene™ fromEni Chem, or as Krynac™ from Bayer, or as Breon™ and Nipol N™ from Zeon.Polyvinyl butyrals are available as Butvar™ from Solucia, as Pioloform™from Wacker, and as Mowital™ from Kuraray. Hydrogenatednitrile-butadiene rubbers are available for example as Therban™ fromBayer and as Zetpol™ from Zeon. Polyacrylate rubbers are available forexample as Nipol AR™ from Zeon. Chloroprene rubbers are available forexample as Baypren™ from Bayer. Ethylene-propylene-diene rubbers areavailable for example as Keltan™ from DSM, as Vistalon™ from ExxonMobile, and as Buna EP™ from Bayer. Methyl-vinyl-silicone rubbers areavailable for example as Silastic™ from Dow Corning and as Silopren™from GE Silicones. Fluorosilicone rubbers are available for example asSilastic™ from GE Silicones. Butyl rubbers are available for example asEsso Butyl™ from Exxon Mobile. Styrene-butadiene rubbers are availablefor example as Buna S™ from Bayer, and Europrene™ from Eni Chem, and asPolysar S™ from Bayer.

Polyvinyl formals are available for example as Formvar™ from LaddResearch.

The synthetic rubbers preferably possess a glass transition temperatureof between −80° C. and 0° C. In addition it is also possible to usemixtures with thermoplastics and elastomers.

The thermoplastic materials are chosen preferably from the group of thefollowing polymers: polyurethanes, polystyrene,acrylonitrile-butadiene-styrene terpolymers, polyesters, unplasticizedpolyvinyl chlorides, plasticized polyvinyl chlorides,polyoxy-methylenes, polybutylene terephthalates, poly-carbonates,fluorinated polymers, such as polytetra-fluoroethylene, for example,polyamides, ethylene-vinyl acetates, polyvinyl acetates, polyimides,polyethers, copolyamides, copolyesters, polyolefins, such aspolyethylene, polypropylene, polybutene, polyisobutene, for example, andpoly(meth)acrylates. The enumeration makes no claim to completeness.

In one preferred embodiment these thermoplastic materials possess aglass transition temperature of between +60° C. and +125° C.

To optimize the adhesive properties and the activation range it ispossible optionally to add bond strength promoter resins and/or reactiveresins. The fraction of the resins is between 75% and 25% by weight,based on the overall mixture of elastomer and resin.

As tackifying resins for addition it is possible to use the tackifierresins that are already known and described in the literature.Representatives include the pinene and indene resins, rosins, theirdisproportionated, hydrogenated, polymerized, esterified derivatives andsalts, the aliphatic and aromatic hydrocarbon resins, terpene resins andterpenephenolic resins, and also C5, C9, and other hydrocarbon resins.Any desired combinations of these and further resins may be used inorder to adjust the properties of the resulting adhesive in accordancewith requirements. Generally speaking it is possible to use all resinsthat are compatible (soluble) with the elastomer; reference may be madein particular to all aliphatic, aromatic, alkylaromatic hydrocarbonresins, hydrocarbon resins based on pure monomers, hydrogenatedhydrocarbon resins, functional hydrocarbon resins, and natural resins.

In a further variant embodiment, reactive resins are added additionallyor alternatively to the elastomer. By reactive resins are meant thoseresins which contain functional groups which under activation,especially thermal activation, are able to enter into chemicalreactions, especially crosslinking reactions. One very preferred groupfrom which the reactive resins can be chosen encompasses epoxy resins.The molecular weight of the epoxy resins varies from 100 g/mol up to amaximum of 10 000 g/mol for polymeric epoxy resins. The epoxy resinscomprise, for example, epichlorohydrin, glycidyl esters, the reactionproduct of epichlorohydrin and p-aminophenol, and/or the reactionproduct of bisphenol A and epichlorohydrin. Preferred commercialexamples are, for example, Araldite™ 6010, CY-281™, ECN™ 1273, ECN™1280, MY 720, RD-2 from Ciba Geigy, DER™ 331, DER™ 732, DER™ 736, DEN™432, DEN™ 438, DEN™ 485 from Dow Chemical, Epon™ 812, 825, 826, 828,830, 834, 836, 871, 872, 1001, 1004, 1031 etc. from Shell Chemical, andHPT™ 1071, HPT™ 1079, likewise from Shell Chemical.

Examples of commercial aliphatic epoxy resins are, for example,vinylcyclohexane dioxides, such as ERL-4206, ERL-4221, ERL 4201,ERL-4289 or ERL-0400 from Union Carbide Corp.

As novolak resins it is possible for example to use Epi-Rez™ 5132 fromCelanese, ESCN-001 from Sumitomo Chemical, CY-281 from Ciba Geigy, DEN™431, DEN™ 438, Quatrex 5010 from Dow Chemical, RE 305S from NipponKayaku, Epiclon™ 673 from DaiNipon Ink Chemistry or Epicote™ 152 fromShell Chemical.

Furthermore, melamine resins as well can be used as reactive resins,such as Cymel™ 327 and 323 from Cytec, for example.

As reactive resins it is also possible, furthermore, to useterpene-phenolic resins, such as NIREZ™ 2019 from Arizona Chemical, forexample.

As reactive resins it is also possible, furthermore, to use phenolicresins, such as YP 50 from Toto Kasei, PKHC from Union Carbide Corp.,and BKR 2620 from Showa Union Gosei Corp., for example. As reactiveresins it is also possible, furthermore, to use phenol resole resins,both alone and in combination with other phenolic resins.

As reactive resins it is also possible, furthermore, to usepolyisocyanates, such as Coronate™ L from Nippon Polyurethan Ind.,Desmodur™ N3300 or Mondur™ 489 from Bayer, for example.

In order to accelerate the reaction between the two components it isalso possible, optionally, to additize crosslinkers and acceleratorsinto the mixture.

Examples of suitable accelerators include imidazoles, availablecommercially as 2M7, 2E4MN, 2PZ-CN, 2PZ-CNS, P0505, L07N from ShikokuChem. Corp. or Curezol 2MZ from Air Products. Additionally suitable ascrosslinkers are HMTA (hexamethylenetetramine) additives.

Furthermore, amines as well, especially tertiary amines, can be used foracceleration.

In a favorable procedure it is also possible, furthermore, to useplasticizers. Here, in one preferred embodiment of the invention, usemay be made of plasticizers based on polyglycol ethers, polyethyleneoxides, phosphate esters, aliphatic carboxylic esters, and benzoicesters. Moreover, aromatic carboxylic esters, relatively high molecularweight diols, sulfonamides, and adipic esters can be used as well.

Additionally it is possible, optionally, for fillers (e.g., fibers,carbon black, zinc oxide, titanium dioxide, chalk, solid or hollow glassbeads, microbeads made of other materials, silica, silicates),nucleators, expandants, adhesion-boosting additives, and thermoplastics,compounding agents and/or aging inhibitors, in the form for example ofprimary and secondary antioxidants or in the form of light stabilizers,to have been added.

In a further preferred embodiment, further additives are added to theelastomer, such as, for example, polyvinyl formal, polyacrylate rubbers,chloroprene rubbers, ethylene-propylene diene rubbers,methyl-vinyl-silicone rubbers, fluorosilicone rubbers,tetrafluoroethylene-propylene copolymer rubbers, butyl rubbers, andstyrene-butadiene rubbers.

Polyvinyl butyrals are available as Butvar™ from Solucia, as Pioloform™from Wacker, and as Mowital™ from Kuraray. Polyacrylate rubbers areavailable for example as Nipol AR™ from Zeon. Chloroprene rubbers areavailable for example as Baypren™ from Bayer. Ethylene-propylene-dienerubbers are available for example as Keltan™ from DSM, as Vistalon™ fromExxon Mobile, and as Buna EP™ from Bayer. Methyl-vinyl-silicone rubbersare available for example as Silastic™ from Dow Corning and as Silopren™from GE Silicones. Fluorosilicone rubbers are available for example asSilastic™ from GE Silicones. Butyl rubbers are available for example asEsso Butyl™ from Exxon Mobile. Styrene-butadiene rubbers are availablefor example as Buna S™ from Bayer, and Europrene™ from Eni Chem, and asPolysar S™ from Bayer.

Polyvinyl formals are available as Formvar™ from Ladd Research.

In a further preferred embodiment, further additives are added to theelastomer, such as, for example, thermoplastic materials from the groupof the following polymers: polyurethanes, polystyrene,acrylonitrile-butadiene-styrene terpolymers, polyesters, unplasticizedpolyvinyl chlorides, plasticized polyvinyl chlorides, polyoxymethylenes,polybutylene terephthalates, polycarbonates, fluorinated polymers, suchas polytetrafluoroethylene, for example, polyamides, ethylene-vinylacetates, polyvinyl acetates, polyimides, polyethers, copolyamides,copolyesters, polyolefins, such as polyethylene, polypropylene,polybutene, polyisobutene, for example, and poly(meth)acrylates.

The bond strength of the heat-activatable adhesive can be increased bymeans of further deliberate additization. Thus, for example, polyiminecopolymers or polyvinyl acetate copolymers can also be used as bondstrength promoter additives.

Release Liner

The release liner used in accordance with the invention has a polyolefinfilm on at least one side.

The release liner to which hot transfer lamination takes place in themethod of the invention is a liner which on thermal activation, andhence inter alia in the hot transfer laminating operation itself,develops a slight pressure-sensitive adhesion. Where the assembly formedof release liner and layer of the heat-activatable adhesive is cooledagain, the adhesion of the two layers to one another is retained. It hasbeen observed that, surprisingly, the heat-activatable adhesive can betransfer-laminated again outstandingly from the release liner onto thedesired substrate to be bonded, and so the substrate can then be bondedas desired.

In the wider sense, the release liners themselves can be regarded asheat-activatable films, which on thermal activation developpressure-sensitive adhesion at least on their surface.

The pressure-sensitive adhesion of the release liners and therefore theadhesion of the release liner layer and of the heat-activatable adhesivelayer may be controlled in particular through the choice of thetemperature in the hot transfer laminating operation; accordingly, thedetachability of the heat-activatable adhesive can be adjusted for theparticular intended application of the diecut that is later present. Asa result of the method of the invention, therefore, it is possible,advantageously, to produce assemblies which are composed of a layer of aheat-activatable adhesive and of a release liner and in which thedetachment force of the two layers from one another can be predefinedand can be adapted to particular profiles of requirements.

It is advantageous to use release liners whose glass transitiontemperature is at least 20° C., preferably at least 30° C., lower thanthe glass temperature of the heat-activatable PSA. Through a suitablechoice of the temperature in the hot transfer laminating operation, therelease liner is then caused to develop its surface pressure-sensitiveadhesion, but the heat-activatable adhesive layer is not yet activatedand therefore is present in a nontacky form.

With very great advantage, coextruded release liners are used. In itssimplest form, the release liner is composed of a coextruded film of apolyolefin and of a hard filmic material such as polyester, PMMA(polymethyl methacrylate), polyimide, polyethylene naphthalate,polyurethane or polyamide. The films are preferably produced bycoextrusion.

In the sense of the invention it is also possible, furthermore, withoutstanding effect, for films laminated together to have been used, or,for example, PE (polyethylene) or PP (polypropylene) may have beencoated subsequently on the hard side, in particular through solution orfrom the melt. In a further form, however, multilayer films may also beused. The condition in each case is that the polyolefin side faces theheat-activatable adhesive.

One very preferred embodiment uses polyolefins, especiallypoly-α-olefins, which in particular possess a glass transitiontemperature which is lower by 20° C., more preferably by 30° C., thanthat of the heat-activatable adhesive. In particular it is possible withadvantage to use release liners of this kind for such heat-activatable,polyolefin-based PSAs.

The polyolefins of the release liner—especially of the coextrudedrelease liner—preferably have glass transition temperatures of +55° C.to +150° C. The bond strength of these polymers can be increased throughdeliberate additization. Thus it is advantageous, for example, to usepolyimine copolymers or polyvinyl acetate copolymers as bond strengthpromoter additives.

To achieve the desired glass transition temperature, here as well again,preferably, the monomers used for producing the release liners, and alsotheir amounts, are chosen such that the desired temperature results whenequation (E1) is applied.

In order to find an optimum range, the molecular weight and thecomonomer composition as well is varied. In order to set a low glasstransition temperature, it is preferred to use polymers having a mediumor low molecular weight. It is also possible to mix low molecular weightpolymers with high molecular weight polymers. Particularly preferredembodiments use polyethylenes, polypropylenes, polybutylenes,polyhexenes or copolymers of polyethylene, polypropylene, polybutyleneor polyhexene.

In a further form the coextruded release liner may have a silicone layeras a repellent layer. This layer is applied on the hard side of thecoextruded film, in other words on the side of the film opposite to thepolyolefin side. This form of the release liner is preferred when, forexample, no liner is used for the diecutting operation and the diecutsare rolled up directly on the coextruded release liner. In that case thediecuts do not stick to the reverse (the siliconized side) of the nextturn.

In a further form of the invention, the coextruded release liner mayhave microstructuring on the polyolefin side. This microstructuring maybe embossed onto the polyolefin side, for example, by means of anengraving roller, with heat. By means of this procedure it is possiblefurther to set the adhesion between the release liner and theheat-activatable adhesive layer, more particularly such that the contactarea between the two layers is reduced and the detachability of theheat-activatable PSAs—more particularly in the form of the diecuts—fromthe release liner is facilitated.

Release liners of this kind are available commercially, for example,from the company Loparex under the trade name FILM PLUS™, and can beused with outstanding effect in accordance with the invention.

In a further variant embodiment of the invention the topography of therelease liner on the polyolefin side is transferred in inverted formonto the heat-activatable adhesive. The topography consists of coherentelevations, which may be either rounded or angular in construction. Theelevations preferably occupy at least 2% of the total area and/or notmore than 65% of the total area. In one preferred embodiment thefraction of the elevations is at least 5%. The remaining area oughtpreferably to be planar. In a further embodiment of the invention,however, the planar area may also have a microroughness, but thismicroroughness should be below the height of the elevations.

By means of a topography of this kind it is also possible further toadjust the adhesion between the release liner and the heat-activatableadhesive layer, in this case, in particular, such that the contact areabetween the two layers is increased and the detachability of theheat-activatable PSAs—especially in the form of the diecuts—from therelease liner is hindered: in other words, higher detachment forces mustbe expended.

The coextruded release liners possess preferably a total layer thicknessof 25-350 μm, more preferably of 50-250 μm. The polyolefin layerpreferably has at least a layer thickness of 2 μm. The polyolefin layerhere ought to be kept as thin as possible, so that in the subsequentdiecutting operation the hard reverse of the release liner, PET, forexample, acts as a hard abutment for the diecutting blades.

Laminating Operation:

After the PSA has been prepared, it is coated initially onto a carriermaterial. This may be a permanently remaining carrier material or elsein particular may be a first temporary carrier material (a substratepresent temporarily). Typically a first temporary carrier material, arelease paper, for example, is used, though this has the disadvantagesdescribed at the outset for the diecutting operation.

Where permanent carrier materials are used, those suitable are thematerials that are customary and familiar to the person skilled in theart, especially non-woven-based materials, since the latter cause lessdisruption to the diecutting operation: such as, in particular, forexample, films (polyesters, PET, PE, PP, BOPP, PVC; HDPE, LDPE).

A feature of the method of the invention, then, is that theheat-activatable adhesive layer is laminated by one exposed side, priorto diecutting, onto a second temporary carrier material, the releaseliner described in more detail above, with accompanying exposure totemperature.

For the purposes according to the invention it may also be advantageousto prepare double-sided heat-activatable adhesive sheets for thediecutting operation. In this case a carrier material is coated on bothsides with the heat-activatable adhesive, and then one of the adhesivelayers is laminated onto the release liner. It may be advantageous hereto provide the other adhesive layer in the interim with a temporarycarrier material or liner material.

For the following hot laminating operation, the release liner oughtpreferably to have a temperature resistance, so that no damage occurs.The requisite temperature resistance and hence the laminatingtemperature are dependent on the activation temperature or glasstransition temperature of the heat-activatable adhesive.

The activating temperature in the hot laminating operation isadvantageously chosen so as to be situated within a temperature rangebetween the glass transition temperature of the release liner and theglass transition temperature of the heat-activatable adhesive. By thismeans it is possible to bring about a situation in which the tack of thesurface of the release liner is brought about, while theheat-activatable adhesive layer continues to remain in a nontacky formduring this stage of the operation.

In one advantageous variant embodiment of the invention theheat-activatable adhesive, present in particular in a form in which itis coated on the first (temporary) carrier, a release paper, forexample, is laminated together with the release liner—in particular,with the coextruded release liner. In this case the polyolefin side ofthe release liner faces the heat-activatable adhesive layer.

In the same way, the adhesive layer present on a permanent carrier canbe laminated onto the release liner.

A first lamination (“preliminary lamination”) may take place over coldrolls (at room temperature—then the release liner is only lying on theheat-activatable film) . After that or, if no preliminary lamination iscarried out, as a “first” lamination, a lamination under activatingconditions (hot lamination) is carried out; in particular by means ofheated rollers, with introduction of heat and pressure. Transfer withpressure takes place, for example, by means of at least one laminatingroll. Rolls used here are preferably rubberized rolls.

For the hot laminating operation the speed, the pressure and thetemperature are set as a function of the activation temperature of theheat-activatable adhesive and of the glass transition temperature of thepolyolefin of the coextruded release liner. Through the choice of thestated parameters it is possible to define the adhesion between therelease liner and the heat-activatable PSA layer.

The hot roller laminator is operated preferably with a pressure rangebetween 1 and 20 bar. The temperature range is preferably between 50 and170° C.; it can be set as a function of the activation temperature ofthe heat-activatable adhesive. Furthermore, different hot rollerlaminators can be combined with one another. The transit speeds arebetween 0.5 and 50 m/min, more preferably between 2 and 10 m/min. Thehot rollers of the roller laminator may be heated from the inside or byan external heat source.

As the laminating speeds go up, the lamination temperature is raised inorder to introduce sufficient heat, which causes the polyolefin layer ofthe release liner, more particularly of the coextruded release liner, tosoften and become tacky.

Diecutting Operation

The method of the invention includes a diecutting operation. Fordiecutting operations of this kind it is possible, to outstandingeffect, to make use, for example, of rotary diecutting. The diecuttingoperation may be a full-cut or kiss-cut operation. Correspondingly it ispossible to carry out the following variants advantageously:

-   -   the diecutting operation severs the adhesive on the release        liner completely;    -   the diecutting operation severs the adhesive on the release        liner incompletely;    -   the diecutting operation severs the adhesive-coated carrier        material completely;    -   the diecutting operation severs the adhesive-coated carrier        material only partly or not at all.

The lamination of the heat-activatable adhesive onto the release liner,more particularly onto the coextruded release liner, and the subsequentdiecutting operation may be carried out advantageously in an inlineoperation, in other words in a combined unit and/or continuous sequence.Alternatively, however, the laminating operation and the diecuttingoperation may also be separated from one another.

Furthermore, it may be of advantage if, prior to diecutting, a liner,made of PET, for example, is laminated onto the open side of theheat-activatable adhesive, in order to protect this adhesive when theassembly is being wound up or is being transported or stored.

Variant A, Continuous Process:

1. Unwinding of the heat-activatable adhesive tape with the releaseliner.

2. Lamination of a siliconized liner film, upstream of the rotarydiecutting cylinder, from above, onto the open side of theheat-activatable adhesive tape.

3. Rotary diecutting process: Severing of the siliconized liner film andof the adhesive assembly. Ideally the diecutting blades perform onlyminimal penetration of the polyolefin layer of the release liner.

4. Stripping: Removal of the matrix net. The diecuts remain on therelease liner.

5. Winding up of the completed products (i.e., diecuts lined with linerfilm and on original release material as carrier) and winding up of thestripped matrix.

Variant B, Continuous Process:

1. Unwinding of the heat-activatable adhesive tape with the releaseliner.

2. Rotary diecutting process: Severing of the adhesive assembly. Ideallythe diecutting blades perform only minimal penetration of the polyolefinlayer of the release liner.

4. Stripping: Removal of the matrix net. The diecuts remain on therelease liner.

5. Winding up of the completed products (i.e., diecuts on originalrelease material as carrier (release liner)) and winding up of thestripped matrix.

Examples of the speed at which the assembly comprising heat-activatableadhesive and release liner, more particularly, coextruded release liner,run through the unit are 0.1 m/min to 100 m/min. Typical speeds fordiecutting operations in the present-day are 10 to 30 m/min.

For further diecutting operations it is also possible in an outstandingway, for example, to use flatbed diecutters. In this case as well thediecutting operation may be a full-cut or kiss-cut operation.

Variant C, Discontinuous Process:

1. Unwinding of the heat-activatable adhesive tape with the releaseliner.

2. Lamination of a siliconized liner film, upstream of the flatbeddiecutting unit, from above, onto the open side of the heat-activatableadhesive tape.

3. Flatbed diecutting process: Severing of the siliconized liner filmand of the adhesive assembly. Ideally the diecutting blades perform onlyminimal penetration of the polyolefin layer of the release liner.

4. Stripping: Removal of the matrix net. The diecuts remain on therelease liner.

5. Winding up of the completed products (i.e., diecuts lined with linerfilm and on original release material as carrier) and winding up of thestripped matrix.

Variant D, Discontinuous Process:

1. Unwinding of the heat-activatable adhesive tape with the releaseliner.

2. Flatbed diecutting process: Severing of the adhesive assembly.Ideally the diecutting blades perform only minimal penetration of thepolyolefin layer of the release liner.

4. Stripping: Removal of the matrix net. The diecuts remain on therelease liner.

5. Winding up of the completed products (i.e., diecuts on originalrelease material as carrier (release liner)) and winding up of thestripped matrix.

Experiments

The invention is described below by means of experiments, withoutwishing any unnecessary restriction to arise from the choice of thesamples investigated.

Production of the Samples

Preparation of the Heat-Activatable Adhesive

EXAMPLE 1

50% by weight of Breon N36 C80 (nitrile rubber) from Zeon, 40% by weightof phenol-novolak resin Durez 33040 blended with 8% of HMTA (Rohm andHaas), and 10% by weight of the phenol resole resin 9610 LW fromBakelite were prepared as a 30% strength solution in methyl ethyl ketonein a kneading apparatus. The duration of kneading was 20 h. Theheat-activatable adhesive was subsequently coated from solution onto agraduated glassine liner (70 μm, Laufenberg) and dried at 100° C. for 10minutes. The layer thickness after drying was 100 μm.

EXAMPLE 2

50% by weight of Nipol N1094-80 (nitrile rubber) from Zeon, 40% byweight of phenol-novolak resin Durez 33040 blended with 8% of HMTA (Rohmand Haas), and 10% by weight of the phenol resole resin 9610 LW fromBakelite were prepared as a 30% strength solution in methyl ethyl ketonein a kneading apparatus. The duration of kneading was 20 h. Theheat-activatable adhesive was subsequently coated from solution onto agraduated glassine liner (70 μm, Laufenberg) and dried at 100° C. for 10minutes. The layer thickness after drying was 100 μm.

Transfer Lamination onto the Coextruded Liner

Examples 1 and 2 were each laminated onto a coextruded PET/PE liner fromLoparex. The liner possesses a total thickness of 90 μm, the PE layerhaving a thickness of about 20 μm. On the reverse (PET side) the linerfeatures siliconization. An easy-release silicone system is applied,with an application of approximately 0.5 g/m² silicone.

Examples 1 and 2 were each run together with the coextruded releaseliner through a hot roller laminator, the coextruded PET/PE liner beingdirected with the PE side facing the heat-activatable adhesive. Thelaminating temperature was 90° C., and both rubber rolls of the rollerlaminator were heated. The pressure was 2 bar and the speed was 1 m/min.Subsequently, before winding, the glassine liner was removed and onlythe heat-activatable adhesive with the coextruded PET/PE liner was woundup.

Overview of the Diecutting Methods Employed:

Rotary Diecutter with Continuous Matrix Stripping.

A rotary diecutter from SMO, Germany, was used. For the different diecutforms, diecutting cylinders from the company RotoMetrics InternationalLtd were used in each case.

The roll width of the adhesive materials used was 130 mm. The diecuttingtrials with the heat-activatable adhesive were carried out bykiss-cutting on the coextruded PET/PE liner. Upstream of the rotarydiecutting cylinder, in some cases a second siliconized auxiliaryrelease material was laminated from above onto the open adhesive side ofthe trial adhesive tape. The auxiliary release material used here was asingle-sidedly siliconized, easy-release PET film with a layer thicknessof 36 μm.

The matrix was stripped at an angle of around 80°. The diecutting speedwas 18 m/min.

Target Product 1:

Square diecuts without connecting bridges. The diameter of the diecutsis 14 mm from tip to tip.

Target Product 2:

Square diecuts without connecting bridges. The diameter of the diecutsis 5 mm from tip to tip.

Target Product 3:

Circular diecuts. The circular diameter of the diecuts is 8 mm.

Target products 4:

Square diecuts with direct connecting edge. The side-edge length of thediecuts is 9 mm.

TABLE 1 Criteria for assessing the fault frequency in the diecuttingtrials. Fault rate Evaluation  0% The matrix was removable with noproblems. In diecutting trials, over 250 linear meters there was not asingle defect, i.e., no diecut was removed during the strippingoperation. 1-99%  Percentage number of defects (missing diecuts) over250 linear meters. The fault rate is based on the total number ofpossible diecuts over 250 linear meters. 100% It was not possible toseparate the matrix from the diecuts. No separated diecuts over 250linear meters.

TABLE 2 Overview of the trial adhesive tapes used, and the diecuttingresults Diecutting method Target Rotary diecutter with continuousExamples products matrix stripping 1 1 0% 2 1 0% 1 2 0% 2 2 0% 1 3 0% 23 0% 1 4 0% 2 4 0%

The diecutting trials demonstrate that all of the products, with thevarious diecut shapes, can be diecut and lattice-stripped withoutproblems on the coextruded PET/PE release liner.

To investigate the further suitability, the circular diecuts (targetproducts 3) were laminated onto an aluminum sheet 1 mm thick at 120° C.with the aid of a hot roller laminator. Following hot lamination, thediecut adhered to the aluminum sheet and the coextruded PET/PE liner wasremovable with no problems. Thereafter the bond strength of examples 1and 2 was ascertained.

The bond strength was measured using 2 aluminum sheets each with a layerthickness of 1 mm, in analogy to DIN EN 1465. The measurement valueswere reported in N/mm². For the adhesive bonding, curing was carried outat 120° C. for 8 minutes in a hot press under 50 N/cm².

The results are listed in table 3 below:

Dynamic shear strength [N/mm²] Example 1 6.5 Example 2 7.7

From table 3 it is apparent that high bond strengths are achievable withexamples 1 and 2.

The invention therefore presents the principle of providing a releaseliner material which, in accordance with its intended use, is typicallymade at least partly anti-adhesive, in other words is intended to avoidthe sticking of adhesive to it, with temporary adhesiveness at least toan extent such that a layer of the heat-activatable adhesive, but onewhich at the time of deposition is itself not tacky, can be depositedadheringly on it. Following deposition, the adhesion of the layers toone another is retained until the heat-activatable adhesive film istaken off again, in particular by transfer to a substrate to be bonded.Surprisingly, the operation of removing the heat-activatable adhesivefrom the release liner is accomplished flawlessly and without residue,and the detachment forces required can be influenced in a defined waythrough the parameters of the method of the invention.

The method differs substantially from the typical procedure ofdepositing heat-activatable adhesives on carriers and bringing about theadhesion from the tackiness of the adhesive.

1. A method of producing diecuts from heat-activatable adhesive films,which comprises a. coating a heat-activatable adhesive initially onto afirst carrier material, b. laminating the heat-activatable adhesivelayer, onto a (release liner) with accompanying exposure to heat, therelease liner having at least one polyolefin-based surface, and thepolyolefinic surface being nontacky at room temperature and being itselfactivatable, with accompanying exposure to heat, in such a way that ithas pressure-sensitive adhesive properties, c. and diecutting theheat-activatable adhesive layer on the release liner.
 2. The method ofclaim 1, wherein said release liner has a glass transition temperatureof at least 20° C. lower than the glass temperature of theheat-activatable pressure-sensitive adhesive.
 3. The method of claim 1,wherein the release liner is an at least two-layer film.
 4. The methodof claim 3, wherein the release liner is a coextruded film comprising apolyolefin layer and a layer of polyester, polymethyl methacrylate,polyimide, polyethylene naphthalate, polyurethane or polyamide.
 5. Themethod of claim 4, wherein the release liner is a film comprising apolyolefin layer and a layer of polyethylene or polypropylene.
 6. Arelease liner for heat-activatable adhesives comprising a layer of aheat-activatable film having at least one polyolefin-based surface whichis nontacky at room temperature and is activatable, with accompanyingexposure to heat, in such a way that it has pressure-sensitive adhesiveproperties.